1. Field of the Invention
The invention relates to a recording apparatus for recording an information signal onto an optical recording disc such as a glass master disc or the like.
2. Description of Background Information
In the case where a laser beam is irradiated onto an optical recording disc such as a glass master disc or the like and tracks are concentrically or spirally formed and an information signal is recorded, the signal must be recorded so that the distance between adjacent two tracks is always maintained constant. To realize such a recording state, recording of the signal is performed while a carriage, from which the laser beam is irradiated, is translated in a radial direction of the disc at a predetermined feed rate. In FIG. 1, a disc 1 is supported by a turntable 2 and is rotated by a rotary shaft 4 of a spindle motor 3 as rotational driving means. A carriage 5 is arranged so as to be movable in the radial direction of the disc 1 and it generally serves as irradiating means for relaying a laser beam carrying an information signal and irradiating the laser beam onto the disc 1, thereby recording the information signal.
In the figure, a laser length measuring apparatus 6 serving as position detecting means is provided, and a laser beam emitted from a laser light source 61 is reflected by a mirror 62 and enters a light interferometer 63. The light interferometer 63 is constructed of a polarization separating filter, a corner cube prism, etc. (all not shown). A part of the incident laser beam is irradiated on the side of the carriage 5 and is also irradiated on the corner cube prism. Two return beams which have been reflected by the side face of the carriage 5 and the corner cube prism, that is, an interference light is supplied to a carriage position detecting section 64. In the carriage position detecting section 64, by counting the number of light and dark interference fringes occurring in the two return beams, a detection signal is produced by accurately detecting the amount of displacement of the moving carriage 5. The detection signal is compared with a carrier signal of a DC voltage which is given by a carrier signal generating circuit 8 as carrier signal generating means, so that a carriage error signal is produced.
The carriage error signal is supplied to a driving circuit 7 as driving means and is power amplified and a driving signal corresponding to the carriage error signal is supplied to the carriage 5. By a feedback loop constructed as mentioned above, the carriage 5 is conveyed at a rate to completely eliminate the carriage error signal. In the case of conveying the carriage 5 at a constant velocity, there is a proportional relation between time t and the position of irradiation, namely, a position r in the radial direction of the disc 1 as shown in FIG. 2A. The spacing P between tracks, therefore, when the information signal is recorded on the tracks in a range from a point A to a point B on the disc is always constant as shown in FIG. 2B.
In the case where the recording density is made high, however, such a track spacing is narrowed and a deviation of relative position between the disc 1 and the position of irradiation becomes larger than a negligible level due to a deviation of the rotary shaft caused by the vibration of the spindle motor 3. When the deviation of the rotary shaft occurs periodically in synchronism with the rotation of the spindle motor 3, the deviation of the irradiating position also becomes periodic in a predetermined range as shown in FIG. 3A. Consequently, the tracks take the form of eccentric circles as shown in FIG. 3B. However, the track spacing P is maintained constant. In the case where the deviation of the rotary shaft occurs irregularly, on the other hand, the deviation of the irradiation position exceeds a predetermined range as shown in FIGS. 4A and 4B, so that the constant track spacing is not maintained. Accordingly, there arises a problem that it is difficult to record the signal with the constant track spacing being maintained, only by measuring the amount of displacement of the moving carriage 5.